ve.ui.MWTemplateDialog: Implement inferring of template data

Clean up of logic implemented during the template-sprint:

* Store spec inside the content model, directly associated
  with the content-part. This allowed fixing the bug where
  two spec-less template invocations overwrote each other's
  made-up template data due to it using "target.wt" as key.
  The opener now provides the fetcher with a "specId" which
  is set to "part/<id>" for wt-generated template targets.

* Batching is now implemented inside the fetcher instead
  of outside. This allows calling "getTemplateSpecs" inside
  the loop with a dedicated callback for each spec to store
  it in the content.parts[i] object passed by reference.

  It also makes it easier to use by different code paths.
  You call it as much as you like and it will queue up
  naturally through javascript yielding and then make a batch
  request. This is based on the pattern I used in MediaWiki core
  for mw.loader#addEmbeddedCSS.

Follows-up e7af635, da679b7.

Change-Id: I4d7121229d060a96d927585c987a1a81a474b922
Merge "ve.ui.MWTemplateDialog: Implement inferring of template data"